Locations of separate visual pathways for spatial vision and object vision were investigated in men by measuring regional cerebral blood flow (rCBF) with positron emission tomography (PET) and H2150. Three extrastriate visual areas associated with these pathways: an occipital association area associated with spatial visual processing, and object vision, a superior parietal area activated by spatial visual processing, an occipitotemporal area activated by object discrimination. Old subjects demonstrated rCBF activations in the same regions as did young subjects, but also demonstrated activation of ventral areas during spatial vision and dorsal areas during object vision, suggesting less functional separation of these visual systems. Patients with dementia of the Alzheimer type (DAT) demonstrated normal activation of rCBF in occipitotemporal cortex during object vision. In healthy adults, significant age-related differences were found on tests of perceptual reasoning and visual memory, but not on verbal memory and verbal intelligence. In healthy adults, visual memory and the discrepancy between verbal and visuospatial non-memory functions was correlated with right-left parietal rCMRglc assymetry; of regional cerebral metabolic rates for glucose (rCMRglc). A longitudinal study of mildly impaired patients, demonstrated that the appearance of neocortically- mediated neuropsychological impairments follows the appearance of significant neocortical rCMRglc abnormalities. Neuropsychological decline in very early Alzheimer's disease was found to have an early plateau phase followed by steady, linear decline. Adults perform worse on mental abilities tests than do younger subjects. Immediate memory and language function are less affected by age in Down syndrome than are long-term memory and visuospatial function.